What's Better: One Large PC or Many Small Ones?
I agree with Duarte that there is no universal good answer for this. In fact the real answer will depend on what time you ask this e.g. the GTX 10 series brought in a swathe of cheap GPU power where [1] a GTX 1060 is the best bang for your buck in terms of FLOPS/$.

Just bear in mind that if you do decide to opt for a bunch of smaller gpus, the amount of onboard memory will limit the size and complexity of your blender scene.

If you are looking to drastically reduce render times, check out airenderer.com. I used it a couple of times, it is good for most scenes and they claim can reduce your render times by up to 97%...[2] it is more like 90% but still very good.

sources:
[1] geforce.com official FLOPS versus RRP
[2] airenderer.com